U.S. Pat. Nos. 3,773,400 and 3,915,549, and OPTICAL AND HOLOGRAPHIC STORAGE PROPERTIES OF TRANSITION METAL DOPED LITHIUM NIOBATE by William Phillips, J. J. Amodei and D. L. Staebler, RCA Reveiw, March, 1972, Vol. 33, No. 1, describe lithium niobate and lithium tantalate crystals doped with iron and are herein incorporated by reference. Volume phase holograms can be recorded in these crystals by changing the index of refraction of selected areas of the crystals with coherent light.
When a hologram is recorded at temperatures of about 160.degree. C in iron doped lithium niobate crystals ionic compensation occurs at essentially the same rate as the electronic space-charge pattern is formed, or more briefly, the hologram is fixed as fast as it is written. By fixing we mean that the recorded hologram can not be erased at room temperature by exposure to either coherent or incoherent light. If recording is done at lower recording temperatures, slower ionic compensation rates allow build-up of electric fields which in turn lead to spurious optical scattering from imperfections in the recording beam or crystal. This results in an image of lower quality. If recording is done at higher recording temperatures, the effects of thermal erasure are more pronounced. This leads to lower diffraction efficiency. During thermal erasure the thermal ionization of electrons in the trapped electronic space pattern formed during recording causes redistribution of those thermally ionized electrons by diffusion, thus reducing the magnitude of the electronic space charge pattern.